The present invention relates to a method of filling a receptacle with a net weight of substance by means of a filler member disposed to introduce the substance into the receptacle while the receptacle is being carried by a force sensor.
Various methods are known for filling a receptacle with a net weight of substance. Traditionally, the simplest method consists in placing a receptacle on a weighing member, the receptacle itself being disposed beneath a filler member which can be turned on and off under the control of the weighing member as a function of the apparent weight that is measured thereby. The apparent weight comprises not only the empty or xe2x80x9ctarexe2x80x9d weight of the receptacle plus the net weight of the substance contained in the receptacle, but also the force that results from the jet of substance on the surface of the substance contained in the receptacle. This force varies not only as a function of how wide the filler member is open, but also as a function of the viscosity of the substance, such that if the viscosity of the substance varies during filling, the weight measurement performed by the weighing member is falsified and the real net weight of substance introduced into the receptacle at the end of filling is not equal to the desired net weight of substance.
In addition, at the moment at which the filler member is closed, the substance that extends between the filler member and the surface of the substance within the receptacle, referred to herein as the xe2x80x9ctailingxe2x80x9d, is added to the weight of substance finally contained in the receptacle at the end of the filling cycle. The weight of the tailing varies as a function of the diameter of the orifice of the filler member immediately prior to conventional methods, the pressure of the jet during filling and the weight of the tailing therefore need to be compensated for in order to end up with the desired net weight of substance in the receptacle.
French patent 2 679 516 discloses a filling method that consists in servo-controlling the flow rate of the substance to a reference flow rate and in filling for a predetermined fixed length of time that is calculated beforehand by dividing the net weight by the reference flow rate. That method makes it possible to eliminate the effects due to the pressure of the jet on the surface of the substance contained in the receptacle by measuring the instantaneous flow rate over successive time intervals during which the force of the jet of substance on the surface of the substance contained in the receptacle is assumed to be constant. In theory, that method should be highly satisfactory, but in practice servo-controlling the flow rate on the reference flow rate does not enable a real flow rate to be obtained that is exactly equal to the reference flow rate, so it is still necessary to perform compensation by checking the real net weight after filling and modifying the parameters of the servo-control loop for subsequent filling cycles so that the real net weight comes as close to possible to the desired net weight.
French document 2 711 610 discloses a filling method comprising steps of: measuring the instantaneous flow rate of the substance introduced into the receptacle over successive time intervals; calculating the total weight of substance introduced into the receptacle on the basis of the instantaneous flow rate over each time interval; and causing the flow of the substance to stop once the total calculated weight reaches the net weight minus the weight of the tailing. That method presents the advantage of automatically taking account of variations in the instantaneous flow rate when calculating the total weight of substance contained in a receptacle, so that the real net weight of substance contained in the receptacle after the filler member has been closed is, in theory, affected only by variations in the weight of the tailing, with the influence thereof being small. Nevertheless, while the receptacle is being put onto the weighing member and while the jet of substance is establishing itself once the flow has been turned on, the weighing member is subjected to oscillations which falsify the measured instantaneous flow rate and consequently falsify the calculated total weight of substance contained in the receptacle. To mitigate that drawback, it is necessary to filter the signal from the weighing member, at least during the initial stages of filling, by using complex algorithms that significantly increase the cost of implementing that filling method.
An object of the present invention is to propose a method of filling a receptacle that can be implemented with algorithms that are simple, while nevertheless making it possible to obtain high accuracy concerning the net weight of the substance introduced into the receptacle.
In order to achieve that object, the invention provides a method of filling a receptacle with a net weight substance by means of a filler member disposed to introduce the substance into the receptacle while the receptacle is being carried by a force sensor, the method comprising at least one cycle comprising the steps of:
placing the receptacle on the force sensor;
commanding the filler member to start a flow of the substance;
repeatedly measuring the time for which the substance has been flowing since the start-of-flow command was issued;
measuring a value of a signal supplied by the force sensor at least at first and second separate measurement instants;
calculating a mean flow rate of the substance over the time interval between the measurement instants;
making at least one computed evaluation of a filling time on the basis of the calculated mean flow rate and a reference weight; and
commanding the flow of substance to stop when the flow time of the substance is equal to the evaluated filling time.
Thus, the step of calculating a mean flow rate for the substance minimizes the consequence of the oscillations on starting so that an accurate result is obtained without it being necessary to use complex calculation algorithms.
In an advantageous variant of the invention, a smoothed mean flow rate is calculated by selecting measurement instants that are offset from the start-of-flow command sufficiently for the force sensor to be substantially stable over the time interval between the measurement instants. The period of instability at the beginning of the cycle is thus automatically eliminated.
In a preferred implementation of the invention, the method includes an initialization cycle comprising the steps of:
placing an initialization receptacle on the force sensor;
preferably measuring a reference tare weight of the receptacle on the force sensor means and storing the reference tare weight;
commanding the filler member to start the flow of substance;
repeatedly measuring a substance flow time from the start-of-flow command;
calculating at least once the smoothed mean flow rate of the substance;
calculating at least one evaluation of a filling time on the basis of the smoothed mean flow rate and a desired net weight of substance;
commanding the flow of substance to stop when the product flow time is equal to the evaluated filling time;
measuring the weight of the receptacle after filling and deducing therefrom a difference between the desired net weight of substance and the real net weight of substance in the receptacle; and
correcting the reference weight as a function of said difference.
Thus, the initialization cycle implements steps that are substantially identical to those of the cycles that are implemented subsequently for filling other receptacles, such that the reference weight is determined at no extra cost.